Modern RF transceivers tend to make extensive use of digital designs for various functions including integrated circuit communication protocols, calibration algorithms and regulation algorithms. In integrated RF transceivers, digital and analogue modules have to be designed to work correctly together on the same die as part of the same integrated circuit. Many constraints for such designs are as a result of the analogue parts. In particular, the performance of the analogue part of an integrated RF transceiver is very much dependent on the quality of the signals input to the analogue circuitry, which can be affected by noise resulting from the digital part of the transceiver. Any digital commutation spur, or spurious spectral line, that falls into the RF band of interest will be treated by the analogue part as an RF signal, and would be amplified along with the signal of interest. A direct consequence of this would be a poor signal to noise ratio, resulting in poor performance.
A schematic illustration of a receiver part 100 of an integrated analogue and digital RF transceiver is shown in FIG. 1. An RF signal is received at an antenna 101, and the signal is amplified by amplifiers 102, 103, bandwidth limited by a filter 104 and mixed using a mixer 105 having a local oscillator signal LO. A resulting intermediate frequency signal is bandwidth limited by a further filter 106 and an IF output signal IFout is provided, which is then provided to other parts of the circuit for demodulation and processing.
Along with the analogue components 101-106, the receiver 100 comprises digital components 107, which may include components such as a phase-locked loop (PLL), an analogue to digital converter (ADC) and a digital to analogue converter (DAC). These digital components will tend to generate spurious spectral lines, or spurs 108, which can interfere with the analogue components 101-106 at various points.
In such a receiver, when a spur signal 108, or one or more of its harmonic components, at a frequency fspur interferes with the analogue signal in any block before the mixer 105, the spur signal 108 will also mix with the LO signal (having a frequency fLO), resulting in a spur in the intermediate frequency output at a frequency of fspur−fLO. In the case of the receiver 100 being an infradyne receiver, if the IF spectrum is filtered with a low pass filter 106 having a bandwidth fbw—IF, then any spur frequency within the range of fLO−fbw—IF to fLO will be visible in the output IF signal IFout. In the case of a supradyne receiver, this range will be fLO to fLO+fbw—IF.
Although other technologies are possible, area constraints as well as process constraints in integrated circuit designs tend to often lead to consideration of digital designs using CMOS technology. CMOS logic is an asymmetrical logic type, meaning that most commutation spurs remain uncompensated in the power supplies or in the ground. This would not generally be the case with differential logic types such as Current Mode Logic, Emitter Coupled Logic or Source Coupled Logic, but in such alternative types the area required on an integrated circuit tends to be larger.
In conventional circuit designs, CMOS parts need to be isolated from any analogue parts of the integrated circuit, so that the commutation spurs do not fall into analogue blocks, as this can result in undesirable amplified signals. Various isolation techniques are known to address this, such as the use of separate power supplies, deep-N well or triple well isolation, optimising of clock trees or positioning digital parts further away from any critical analogue blocks such as voltage controlled oscillators (VCOs) or low noise amplifiers (LNAs). The use of a well defined power supply strategy can also ensure that there is no undesirable current loop that could eventually be closed through the PCB wires back into RF grounds or supplies.
However, RF transceivers sometimes have to deal with very small input power signals, making them more sensitive to spurs. Even when using isolation techniques such as those mentioned above, digital spurs falling into the analogue RF signal processing chain can be a problem, as the ratio between the RF signal to the spur signals may be too high.
Other techniques to reduce or mitigate the spurs are also known, such as spreading clock frequencies via FM modulators, sigma delta modulators, or with randomizing techniques that tend to spread the harmonics. These techniques, however, may be insufficient by themselves to reduce the effect of spurs below a desired level. Other techniques for reducing the effect of spur signals are consequently also required.
It is an object of the invention to address one or more of the above mentioned problems.